Cucumber grower has it's doubts about diffuse glass

This article is sent in by Dutch cucumber grower Sjaak van Dijk, who has it's doubt on the use of diffuse glass in greenhouse horticulture. A reply on this article by researchers from Wageningen University Horticulture Extension can be found here.

In practice, the terms diffused glass and diffused light are frequently interchangeable. Here we are talking about diffuse light, and what we want to know is: does it actually provide additional yield?

The effect of diffuse light

Measurements by EARS in Delft, The Netherlands (EARS specializes in photosynthesis measurements) shows a marginal effect on the assimilation seen in the lower leaves. So apparently, diffuse light on (for example) a vegetable crop has no noticeable production increase as a result.

So why is diffused glass still recommended?

It all depends on what you are comparing it to. In the tests, diffused glass with AR coating and anti-condensation effect is compared to untreated standard glass. In this equation, if conducted properly, diffuse glass in fact leads to a 5% loss of light. Several factors however, make the comparison tricky. The loss of light through diffused glass is compensated by an AR coating (5% more light) and anti-condensation (7% more light), resulting in 12% more light in the greenhouse. When we subtract the earlier 5%,we end up with a meagre 7% light gain, wrongly attributed to diffuse glass.

Diffuse glass, by the way, also leads to a 5% increase of heating costs, because less light means less heat .

Clean glass prevents light loss by condensation

Really clean glass has a "hydrophilic" surface. This means that water drops on clean glass form a unified film, again blocking the light. Only treatment with an etching cleaning agent like fluoride or hydroxide is able to really do the job. Condensation begins at the gutter and will eventually fog the entire window in autumn.

Old-fashioned glass still the best choice

By spraying the greenhouse with a fluorine-containing medium (light etching) in autumn, the glass will retain a pure surface for 3-4 months. Contamination by dust, algae and other culprits will cause condensation to return. Therefore, investing in coated glass to compensate for the disadvantages of diffuse glass without proper cleaning, seems to make little sense. The combination of coated glass and rail truss offers a feasible solution.

By: Sjaak van Dijk / Klimrek